Japanese patent application Kokai publication No. 62-42716 discloses as an antibacterial electret material a filter material formed by subjecting the surface of a fiber to an antibacterial treatment with an antibacterial agent and further subjecting the treated fiber to a treatment for imparting an electret property. In this filter material, however, the volume resistivity of the surface of the fiber was reduced because the antibacterial treatment was conducted by using an organic antibacterial agent and a surfactant for dispersing the antibacterial agent. For this reason, the filter material had a problem that, even after the treatment for imparting an electret property, the quantity of electric charge was small and the charge decayed rapidly.
Japanese patent application Kokai publication No. 3-186309 discloses an air filter comprising an antibacterial nonwoven fabric composed of a fiber and, added thereto, an inorganic antibacterial zeolite and an electret nonwoven fabric laminated on the antibacterial nonwoven fabric. Although this air filter could suppress the propagation of trapped bacteria on the side of the antibacterial nonwoven fabric, it was disadvantageous in that, on the side of the electret nonwoven fabric, trapped bacteria propagated and emitted an offensive odor or rescattered to contaminate the atmosphere of a room. Further, since the electret property was not imparted to the air filter as a whole, the performance of the filter lowered.
Japanese patent application Kokai publication No. 62-42715 discloses a composite material comprising an electret nonwoven material and, superimposed thereon, a nonwoven fabric subjected to an antibacterial treatment. This composite material, however, had a similar drawback.
Studies conducted by the present inventors revealed that the process wherein the formed nonwoven fabric was subjected first to an antibacterial treatment and then to a treatment for imparting an electret property had a fundamental problem that it was difficult to impart a good electret property and, further, since the process time was lengthened, the product cost was increased.
On the other hand, the process wherein the formed nonwoven fabric was subjected first to a treatment for imparting an electret property and then to an antibacterial treatment had a drawback that the antibacterial treatment deteriorated the electret performance and a problem that the process time was similarly lengthened and hence the product cost was increased.